This invention relates to an inverter apparatus for converting direct current electric power into alternating current electric power, and more particularly to an inverter apparatus for improving the operation frequency boundary.
FIG. 1 is a schematic diagram illustrating a power inverter apparatus wherein an example of a well-known brushless motor application is shown. Reference numeral 10 represents a DC power source to provide the output current I.sub.D. Power inverter 11 includes thyristor-branches UP, VP, WP, UN, VN and WN connected in a bridge configuration. A DC reactor 12 absorbs ripple voltages of the DC power source 10 and the power inverter 11 to smooth the DC current I.sub.D. A synchronous motor 13 is provided with an armature winding 14 and a field winding 15. A position detector 16 detects the angle of revolution of synchronous motor 13. Reference numeral 17 represents a triggering control circuit that controls triggering thyristor-branches UP, VP, WP, UN, VN and WN in accordance with the output signals from position detector 16.
FIG. 2 is a waveform diagram illustrating the operation of the power inverter apparatus shown in FIG. 1. In FIG. 2, E.sub.U, E.sub.V, and E.sub.W represent the induced voltages of the respective phases U, V and W of synchronous motor 13, and I.sub.U, I.sub.V and I.sub.W respectively represent the currents of phases U, V and W of synchronous motor 13. The operation of the power inverter of FIG. 1 will next be described when the commutation is such that first the thyristor-branches UP and WN are in a conductive state and then the thyristor-branch VP is conductive. In FIG. 2, a triggering pulse is supplied to the thyristor-branch VP at time t1. Since the voltage E.sub.U is greater than the voltage E.sub.V at time t1, a voltage of (E.sub.U -E.sub.V) is applied to the thyristor-branch VP as a forward voltage, so that the thyristor-branch VP will be turned on. At the same time, a current shown by a dotted arrow A in FIG. 1 flows. Consequently, the current I.sub.U decreases, in turn, the current I.sub.V increases. At time t.sub.2, when the current I.sub.U becomes zero, the thyristor-branch UP will be turned off. At this instant, since the voltage E.sub.U is greater than the voltage E.sub.V, a voltage of (E.sub.U -E.sub.V) is applied to the thyristor-branch UP as a reverse voltage. At time t.sub.3, when the voltage E.sub.U becomes equal to the voltage E.sub.V, the reverse voltage applied to the thyristor-branch UP becomes zero, and thereafter the voltage applied to the thyristor-branch UP is reversed in polarity. The forward voltage is applied to the thyristor-branch UP. Therefore, it is necessary that the thyristor-branch UP should completely be turned off by time t.sub.3 so that the capability of forward voltage blocking has been recovered. This requires that the period between the times t.sub.3 and t.sub.2 should be greater than the turn-off time of a thyristor element.
Greater capacities and higher speeds over such prior circuits are now desirable. For example, there is a need for a compressor conventionally driven by a gas turbine to be substituted with a brushless motor provided with features such as simplified operation, easier maintenance, high efficient and economical performance, and superior control characteristics. In such applications, a brushless motor must be provided with both greater capacities and higher speeds, such for instance as 30,000 kW and 6,000 RPM or 10,000 kW and 9,000 RPM and further to be operated at higher voltages such as from 3 kV up to 14 kV. This necessitates a power inverter apparatus capable of generating higher voltages with greater power capacities and higher frequencies.
In general, thyristor elements tend to have long turn-off times when provided with characteristics of higher voltages and greater current capacities, and so no high speed thyristor suitable for such applications has been developed. For instance, in the case of a thyristor of 4 KV both in peak repetitive off-voltage and in peak repetitive reverse voltage with average on-current of 1,500 A, the turn-off time is approximately 400 .mu.sec. Here, a thyristor having a turn-off time of less than 250 .mu.sec. is specifically selected for use and will be described below.
In FIG. 2, it is desirable to minimize the electrical angle from time t.sub.2 to time t.sub.3 so as to enhance the power factor of synchronous motor 13 and to decrease the torque ripples. For example, the case when the time t.sub.2 to time t.sub.3 is controlled to 15.degree. or less will be described. The turn-off time of a thyristor element is 250 .mu.sec. By taking variations of control into consideration, if the period from time t.sub.2 to time t.sub.3 is controlled to a target of more than 500 .mu.sec, the minimum value of the period T.sub.1 at the output side of inverter 11 is expressed as follows: EQU T.sub.1 =500 .mu.sec.times.360.degree./15.degree.=12000 .mu.sec=12 msec(1)
therefore, the maximum value f.sub.1 of the output frequency will be EQU f.sub.1 =1000/12=83.3 Hz (2)
Therefore, the number of revolutions of the brushless motor is limited to 2500 RPM in case of a four-pole machine or to 5000 RPM even in case of a two-pole machine. An appropriate speed-increasing gear mechanism is required to obtain necessary higher speeds. However, a speed-increasing gear mechanism designed for greater capacities and higher speed revolutions has many difficulties in manufacturing, so that the use of a brushless motor for such applications has inevitably been limited.